Transformer with five-leg core



Jan. 29, 1957 w. M. JOHNSON ET AL 2,779,926

TRANSFORMER WITH FIVE-LEG CORE Filed Jan. 25, 1954 Z af ffm/f7.

United States Patent O TRANSFORMER WITH FIVE-LEG CORE Wallace M. Johnson and Stanley B. Williams, Pittsfield,

Mass., assignors to General Electric Company, a corporation of New York Application January 25, 1954, Serial No. 405,912

6 Claims. (Cl. 336-5) This invention relates to a tive-leg magnetic core. More particularly the invention relates to a three-phase transformer with a five-leg core arranged for the efficient use of core materials.

As the transmission voltages of electric power have increased and the output of generating equipment has increased, there has been a demand for larger and larger power transformers to handle these steadily increasing loads. Railroad clearances have limited the height of power transformers to be shipped by rail and in recent years efforts have been directed toward increasing the capacity of power transformers Without increasing their overall height.

One type of transformer which has provided increased KVA without an increase in height has utilized a magnetic core with ve legs with windings on just the inner three legs. This has reduced the yoke height of the core thereby providing a tive-leg transformer with greater window height than a conventional three-leg transformer having the same overall height.

It is an object of the present invention to provide a veleg transformer which not only has the same KVA rating with less overall height than a three-leg-core transformer but has a core weighing less than a conventional three-leg core with the same KVA rating.

lt is another object of the invention to provide an efiicient five-leg core in a three-phase transformer having a harmonic llux suppression winding.

Other objects of the invention will be apparent from the following specification taken in connection with the accompanying drawing in which Fig. l is a diagrammatic view of a three-phase transformer constructed in accordance with this invention; Fig. 2 is a vector diagram of the fluxes in a live-leg core constructed in accordance with this invention; Fig. 3 is a graph illustrating the saving in core weight in a transformer constructed in accordance with Fig. 1.

Briefly stated, in accordance with one of its aspects, this invention is directed toward a tive-leg magnetic core assembly comprising a main core including a center leg and two side legs defining two main core windows having similar rectangular profiles, and a side core extending outward from each of the side legs, the side cores having the same window height as the main core windows and having a cross-sectional area between 1/10 and 1/2 the crosssectional area of each side leg.

Referring to Fig. l, the magnetic core is seen to consist of a center leg and two side legs 11 and 12 positioned between a pair of yokes 13 and 14, two sections, 13a and 13b, being indicated for the yoke member 13. Extending outward from each of the side legs 11 and 12 is a side core 15 and 16 respectively. The entire ve-leg structure including the three main legs 10, 11 and 12 and side cores 15 and 16 is preferably composed of a magnetically oriented material such as silicon steel in llaminated form. The side cores 15 and 16 need not be of the same thickness as the rest of the core as long as their cross-sectional area bears the relationship to the core to be described hereinafter. Each of the legs 10, 11 and 12 has a magnetic winding 17, 18 and 19, respectively. While only a single winding is shown for each leg, there may be in actual practice more than one such winding on each leg.

In order to prevent a third-harmonic iluX from circulating through the yoke members 13 and 14 and side cores 15 and 16, a flux control winding 20, which is short-circuited, is wound around various portions of the side cores 15 and 16 and the yoke member 13. A plurality of arrows indicating the direction of flux flow at a particular instant are shown on Fig. l. These arrows have reference to the vector diagram illustrated in Fig. 2.

Referring to Fig. 2, the balanced three-phase uxes in the legs 10, 11 and 12 are represented by the three sides of an equilateral triangle which are designated by the numbers 10, 11 and 12 to represent the ilux in the similar- Aly numbered parts of Fig. l. Other numbered vectors of the diagram relate to the flux in similarly numbered parts of Fig. 1. The yoke uxes are represented by vectors which connect from the corners of the triangle to a common point inside the triangle. This point may be located any place along a line from the center of the triangle to the apex. lf it is located at the apex, the iluxes in the outside cores are zero and the core becomes a three-leg core. If the point is located in the center of a triangle the core is a conventional tive-leg core with side cores and top and bottom yokes both having 57.7% of the crosssectional area of the main legs 10, 11 and 12. For an intermediate position of the common point, the other cores 15 and 16 have a lesser cross-sectional area than in the conventional tive-leg core and it is preferred that the crosssectional area of the side cores 15 and 16 be from V10 to V2 the cross-sectional area of the main legs 10, 11 and 12. Within this range it is preferred that the side cores have a cross-sectional area of about three-tenths that of the main legs. Any decrease in cross sectional area of the side cores 15 and 16 is compensated by an increase in the cross sectional area of the yoke members 13 and 14 as determined by the relationship shown in Fig. 2. The relationship shown in Fig. 2 represents the cross-sectional areas of the associated core members, since in order to determine the core configuration having the optimum weight for given core characteristics a constant flux density must be assumed for each core part. Then from the triangle of Fig. 2, the ratio A of the cross-sectional area of a yoke member 13 or 14 to the cross-sectional area of a main leg 10, 11 or 12 is substantially equal to:

where B is the ratio of the cross-sectional area of a side core 15 or 16 to the cross-sectional area of a main leg 10, 11, or 12.

The effect of the cross-sectional area relations mentioned above may be seen in the graph of Fig. 3. In Fig. 3, the per cent of weight of a typical core is plotted against the ratio of the cross-sectional area of outer cores such as 15 and 16 to the cross-sectional area of the main legs such as 10, 11 and 12. At a ratio of zero, the core becornes a conventional three-leg core having an arbitrary weight of At a ratio of 0.58, which is the ratio of a conventional fiveeleg core, the total core weight is approximately 101.5%, which is slightly greater than the weight of a standard three-leg core. Between the range of 0.1 and 0.5 the weight is less than for either a conventional three-leg or a conventional ve-leg core, and at a ratio of 0.3 the weight of a five-leg core constructed in accordance with this invention is approximately 95.3% of the weight of a conventional three-leg core of the same capacity. ln a transformer weighing many tons, this saving in weight is appreciable. The curve of Fig. 3 is typical but for different core proportions the curve will vary.

Witha ve-legcoreQthere. exists two paths in the yoke idr. the. uxirom. each. leg, .andhecause ,cime-.saturation of the iron, the division between paths will vary during each cycle. The net result'is that there will exist a large .third .harmonie oLiiuxwhichcircmates in..the yoke Yand .thefside .cpresbutdoesnot enterthemainlegs. .Theshortlnircuited iiux control .winding20 `consists of a .cable Wound .aroundithe .yoke with Vthe turns .ineach Yphase adjusted to force the desired iiux division as indicated by Fig. 2. Anumber =ofturns in. each vphase is. used such. that when .the .desired flux distribution is :obtained Vthe fundamental lfrequency voltages induedin the winding add up to Zero, TheA core willfunction without. theux-control. winding .butit,hasbeen-.foundthat .suppression of the third- .harmoniouxreducesthe noise. produced .by .the-.transformer.

While the present invention has. been described with referencetoparticularembodimentsthereof, it will be understood ,thatnumerous modifications may be made by .those skilledinthe artwithout actually departing from the invention. Therefore, weairnin the appended claims .to coverall such equivalentvariations ascome within .the vtrue spiritand. scope of the, foregoing disclosure.

`What weclaim as new and desire to secure by Letters Patent. of the UnitedStates is:

1 1,.l A.iiveleg magnetic core assembly comprising three main Core .legs and two yoke. members, said legs being positioned at spaced intervals .between said pail-.of yoke gnernbers, said. .three legs and two yoke members defining apairof main core .windows of similar proiile, and a .sidente-extending outward from each of the outer main .core.legs, the side. cores having the same window height as .the `two main core windows ,and Vhaving a cross-sectional area of between one-tenth and one-half .the cross-sectional .area of each of the main core legs, .the ratio A of the crosssectional area of eachof said yoke members to the crosssectionalarea of each of said core legsbeing. substantially Where B is a ratio of the cross-sectional area of each of said s idecore members to the cross-sectional area of each ofsaid yoke members.

2. kA `five-leg magnetic core assembly comprising a main Gore including a center leg and two side legsv dening two main core-windows having similarrectangular profiles, and aside core extending outward from each of said side legs,the .side cores havingthe same window height as the main. Core windows and havingV a cross-sectional area between One-tenth and one-half the cross-sectional area of each side leg,.the ratio A of the cross-sectional area of each f .Said yoke members vto .the cross-.sectional area of each of said core legs being substantially A;-. =\%(0.'5.),2+ (0.'866-1?)2 Where B. isythe, lratioof cross-sectional area of each. of saidtside core .members to .the cross-sectional area of each of said yoke members.

l 3. A live-leg magnetic core assembly comprising a main core including a center leg, two side legs and a pair of yokemembers defining two main core windows having .Similar rectangular proles, and a side core extending outward from each of said side legs, the side core windows .having..the.sameheightas.the main core .windows andsaid side cores having a crossjsectional area of about onethird the cross-sectional area of each side leg, the ratio A of the cross-sectional area of each of said yoke members to the cross-sectional area of each of Said core legs being substantially whereI 1,37. isthe ratio of. cross-sectional area of each of said side core members to the cross-sectionalareaof each of said yoke members.

4. A tive-leg magnetic core assembly comprising amain core including a center leg,` two `side legs and ay pair of yoke members deiiningtwo main core windows having similar rectangular proles, aside core of rectangular prole extending outward from each of said side legs, the side cores having the same window height as the main core windows and having a cross-sectional area-ot between one-tenthand .one-half the.crosssectional area of each sideleg, and a short-circuited flux control winding around at least one of said yoke members of Asaid assembly, the ratio A of-.the cross-sectional area of each `of said yoke membersto. the cross-sectional area of each of said core legs .being substantially where B is the ratio of cross-sectional area of each of said side core members to the cross-sectional area of each Y of Asaid yoke members.

5. A tive-leg magnetic core assembly as claimed in claim 4 wherein the cross-sectional area ofthe side cores is about one-third the cross sectional area of each side leg.

6.A tive-leg three phase transformer comprising a main magnetic core including a center leg, two side legs and arpair of yoke members defining two main core windows having similar rectangular profiles, a side core of rectangular profile extending outward from each of said side legs, the side cores having the same window height asthe main core windows and having a cross-sectional area of about one-third the cross-sectional area of each side leg, at least one inductive winding on each of the three legs of the main magnetic core, and short-circuited ux control winding around the yoke members of said transformer on one side of 4said inductive windings, the ratio A of the cross-sectional area of each of said yoke members to the cross-sectional area of each of said core legs being substantially where B is lthe ratio of cross-sectional area of each of said side c oremrnembersY tothe cross-sectional area of each of Said reke. .members- References Cited inthe file of this patent UNITBDSTYATES PATENTS 

